Modern imaging methods are frequently used to generate two- or three-dimensional image data that can be used to visualize an imaged object to be examined and in addition also for further applications.
Imaging methods are frequently based on the acquisition of X-rays, wherein so-called projection measurement data is generated. For example, projection measurement data can be acquired with the aid of a computed tomography system (CT system). In CT systems, typically a combination of an X-ray source and an oppositely arranged X-ray detector arranged on a gantry rotates around a measuring chamber in which the object to be examined (referred to hereinafter without restricting generality as the patient) is located. In this case, the center of rotation (also known as the “isocenter”) coincides with a so-called system axis z. During the course of one more revolutions, the patient is irradiated with X-rays from the X-ray source wherein projection measurement data or X-ray projection data is acquired with the aid of the opposite X-ray detector.
The X-ray detectors used for CT imaging typically have a plurality of detection units which in most cases are arranged in the form of a regular pixel array. Each of the detection units generates a detection signal for X-rays incident on the detection units, said detection signal being analyzed with regard to intensity and spectral distribution of the X-rays at specific times in order to obtain conclusions regarding the object to be examined and to generate projection measurement data.
Other imaging techniques are based on magnetic resonance imaging. During the generation of magnetic resonance images, the body to be examined is exposed to a relatively high basic magnetic field of, for example, 1.5 tesla, 3 tesla, or in newer high magnetic field systems, even 7 tesla and more. Then, a suitable antenna device emits a radio-frequency excitation causing the nuclear spins of specific atoms that are excited to resonance by this radio-frequency field in the magnetic field to be flipped by a specific flip angle relative to the magnetic field lines of the basic magnetic field. The radio-frequency signal radiated on the relaxation of the nuclear spins, the so-called magnetic resonance signal, is then detected with suitable antenna devices, which can also be identical to the transmission antenna device. The raw data acquired in such a manner is finally used to reconstruct the desired image data. For spatial encoding, respective defined magnetic field gradients are superimposed on the basic magnetic field during the transmission and readout or reception of the radio-frequency signals.
The imaging methods are not only suitable for the imaging reproduction of anatomical structures. In addition, work is increasingly being performed on functional imaging by means of the above-described imaging methods with which functional or dynamic measured variables can be determined, such as, for example, the measurement of the blood flow rate in blood vessels.
In the visualization of functional relationships and also patients' body structures, so-called contrast media are used in medicinal imaging. However, before contrast-medium-supported medical imaging can be started, it is necessary to ensure that, after the injection of the contrast medium into the patient's body, the contrast medium is also located in the region of interest of the patient's body. One possibility for visualizing the distribution of the contrast medium in the body consists in the performance of a so-called bolus-tracking scan (BT scan in short), which is performed before the actual imaging. A BT scan of this kind can entail a time-dependent CT image with low resolution, with which a time-density curve of a sub-area of a region of interest is acquired. Usually, such a sub-area for a BT scan includes a slice, which is embodied orthogonally to the z-direction, the direction of the system axis of the imaging system, and is also considered. Specifically, with the BT scan, attenuation values are acquired as a function of time and space in a sub-area of the region of interest in which generally an artery is located. If the injected contrast medium now flows through the observed artery, the attenuation values are significantly increased. If a predetermined threshold value for the attenuation values is exceeded, for example 150 Hounsfield Units (HU), this can be interpreted as meaning that the contrast medium is present in the region of interest in a sufficient concentration and the actual examination of the image started. The position and size of the sub-area examined with the BT scan can usually also be amended manually.
However, manual adaptation and localization of the sub-area is only effective if the anatomical structures can be clearly identified on a previously compiled overview image (see FIG. 1). If, however, the decisive structures are very small and difficult to differentiate, such as, for example, in the case of blood-carrying arteries in the neck region, identification solely on the basis of this overview image can be very difficult. In such cases, however, it is nevertheless necessary to define a sub-area for the BT method. The sub-area is then generally located outside the patient so-to-speak “in the air” and the distribution of the contrast medium is instead monitored via corresponding imaging by direct observation of the patient. If the operator has the impression that the contrast medium is present in the region of interest in a sufficient concentration, the actual imaging is started manually.
A procedure of this kind requires the user to have experience and is in addition not particularly precise. Often, the time for starting the imaging is set too late so that the total time for which the contrast medium is located in the patient is extended. However, in principle, it is attempted to achieve the shortest possible dwell time of the contrast medium in the body since the contrast medium can be stressful for the human body. Starting the imaging too early can result in a deterioration of the image quality. In the most unfavorable case, it is even necessary to repeat the imaging and the administration of the contrast medium, which is an additional stress for the patient.